The `python_common` role is very slow

[gregharvey]

At the moment we have a role, debian/python_common, which ensures any system packages we do not want are removed and installs a set of common python packages using pip. This role gets included a few times, but it shouldn't run every time - it should be sufficient to run it once per playbook, or not at all if the correct versions of software are already in place. It could do with refactoring, as it's a major performance hit.